There has long been a great need to enhance data transmission speed in wireless communications. To address this, work on next-generation data communication standards such as Long Term Evolution (LTE) has been proceeding. In high-speed data communication standards such as LTE, Multiple Input Multiple Output (MIMO) technology has been attracting attention since it can increase bandwidth in a simulated manner by using multiple antennas at both the transmitting and receiving ends for transmission and reception of signals. Among communication systems using MIMO technology, a closed-loop communication system is one in which information obtained at the receiving end is fed back to the transmitting end in order to achieve efficient data transmission in accordance with the communication environment or reception conditions. Based on the information received from the receiving end, the transmitting end determines the combination of antennas to be used for data transmission.
For example, in the multiple antenna system disclosed in Japanese Laid-Open Patent Publication No. 2008-61253, the receiving apparatus generates feedback information and transmits the feedback information to the transmitting apparatus. The feedback information is generated based on maximum channel quality, an antenna group index associated with the maximum channel quality, rank information, and remaining channel quality information associated with the rank information. On the other hand, the transmitting apparatus selects either multiple user mode or single user mode, based on the feedback information received from the receiving apparatus. Then, the transmitting apparatus transmits via a plurality of antenna groups or a single antenna group the user data streams selected in accordance with the selected transmission mode.
On the other hand, in the communication system disclosed in Japanese Laid-Open Patent Publication No. 2007-20047, when the receiving apparatus detects a state change from normal to abnormal or from abnormal to normal in any one of its receiving channels, the receiving apparatus calculates the number of transmitting channels for the transmitting apparatus so that the number of transmitting channels does not exceed the number of normally operating receiving channels. Then, the receiving apparatus transmits transmitting-channel information indicating the thus calculated number of transmitting channels to the transmitting apparatus. When the transmitting-channel information is received, the transmitting apparatus adjusts the number of operable transmitting channels so that it matches the number indicated by the transmitting-channel information.